Grid test fixtures, also commonly known as "grid translator fixtures," are used in conjunction with automated, computer-based testing equipment to test the functioning of printed circuit boards. In the process of testing the circuit boards, the test fixture serves as a framework structure that facilitates the establishment of an electrical contact between test points on the circuit board being tested on the one hand, and the testing equipment on the other hand. The number of circuits to be tested on any given circuit board can be quite large, numbering in the tens of thousands, and there typically is a switch for each individual test point. During the testing process the test equipment transfers test signals to selected circuits on the circuit board that is being tested, and a pass, no pass result is obtained. In this way the proper functioning of a circuit board can be quickly checked and verified.
Test fixtures of this type typically include a series of parallel, spaced apart plates, each having a plurality of test probe holes drilled therethrough in a predetermined specific pattern that corresponds to the pattern of test points on the circuit board to be tested. The plates are typically manufactured of a plastic material such as Lexan, G10 or FR4. Test probes, also called "test pins" or "translator pins" extend through the test probe holes in the test fixture plates. The test probes are generally spring-loaded and are used to establish electrical contact between test points on the circuit board on one side of the test fixture, and switches interconnecting the probes to the test analyzer on the opposite side of the test fixture. Because the array pattern of the test points on the circuit board is different from the array pattern of the test analyzer, many of the test probe holes drilled through any one plate in the test fixture will be in a slightly different position from the corresponding test probe holes drilled through the next adjacent plate. This results in the test probes being arranged in an image pattern on the circuit board side of the test fixture, and a gridded pattern on the opposite side of the test fixture. Many of the test pins are thus tilted in the test fixture in an angular orientation relative to the plane of the plates such that the pins are inclined at oblique angles relative to the plate plane. This has led to the test pins sometimes being called "tilt pins." Given the many tens of thousands of test probes that may be included in a test fixture, the positioning of the test probe holes drilled through the plates must be precisely controlled. This is typically done with sophisticated computer controlled drilling equipment. But it is also critical that the test fixture is assembled in a controlled and precise manner to minimize any errors in the alignment of the plates. Thus, the plates must be properly oriented with respect to one another so that the test probe holes from one plate to the next are precisely aligned so that the test probes correctly fit through the holes.
In the testing process the circuit board that is to be tested (i.e., the board under test) is brought into physical contact with the grid test fixture such that the ends of the test pins, which extend beyond the upper plate in the test fixture, are brought into physical contact with associated test points on the board under test. The tip of each test pin contacts, or "probes" a specific associated test point on the board. The spring probes of the test equipment, which as noted are in a regular grid array, are electrically interfaced with the test pins. As well known in the art, and as noted above, the test probes are resilient, typically with spring loaded tube-in-tube arrangements. Typically the board under test is sandwiched between a pair of grid test fixtures, one probing each side of the board, so that both sides of the board are tested. As the test equipment is brought into contact with the test probes a compressive force is applied to the test probes, compressing each probe so that each makes positive contact with the test point (through the test pins) on the board under test that is associated with the probe. The compressive force applied to the test fixtures insures good electrical interconnectivity between points on the test machines, through the test probes and the test pins to the associated test points on the board under test.
The testing equipment may apply a substantial compressive force to the test fixtures, and thus on the board under test. With some test equipment up to 8 tons of hydraulic force is applied to the test probes. While a substantial compressive force helps ensure good electrical interconnectivity between the test machine and the board under test, it also can cause significant problems. One such problem occurs when the compressive force causes the plates in the test fixtures to be forced out of their ideally aligned positions relative to one another. As noted above, many of the test pins in the test fixtures are oriented in tilted positions relative to the plane of the plates in the test fixture. In other words, many of the pins are at oblique angles with respect to the plane defined by the parallel plates in the test fixture. Occasionally the orientation of test pins (which is dictated by the positioning of the test points on the board under test) calls for many of the test pins to tilt in the same general direction. When a substantially greater number of test pins tilt in one direction over another, the ideal alignment of the test plates in the fixture can be "skewed" when the test fixture is compressed in the test equipment. In other words, under compressive force the plates in the test fixture move relative to one another, shifting them out of their original, aligned position. This skewing is caused by the uneven forces applied to the fixture as a result of the large number of pins tilting in one general direction.
Test fixture skewing causes several problems, including test pin binding, and possible faulty electrical probing due to misaligned test pins. Significant skewing can also cause substantial problems for the hydraulic rams that compress the test fixtures, resulting in expensive repair problems.
In one traditional method of assembling grid test fixtures, the test fixture is assembled with a series of posts spaced around the periphery of the fixture that secure and separate the plates. The posts are constructed of a series of plastic or metal spacers that are inserted between the plates. The spacers separate the plates and hold them in a parallel array. Hollow rods are inserted through bores through the spacers to hold the spacers in the proper orientation. As described above, test fixtures are assembled so that they can be inverted. This allows both sides of the circuit board to be tested. As such, with such traditional assembly arrangements, long connectors such as threaded bolts are typically inserted through the rods in order to hold the entire test fixture together in a fixed position. This manner of assembling test fixtures has several limitations. First, several different sizes of spacers are required because although the plates are parallel, they generally are not all evenly spaced from one another. This results in an increased number of parts that must be kept in inventory. Further, variances in the thickness of the spacers and the plates as a result of manufacturing tolerances for those parts can lead to misalignment of the plates when the test fixture is assembled. Since in any test fixture there are multiple plates, the cumulative effect of size variances in the spacers and the plates can lead to the plates being assembled in a non-parallel orientation. This in turn can lead to misalignment of the test probe holes between plates. Finally, assembling a test fixture in this way can be a time-consuming and tedious job.
An improved method of assembling test fixtures is disclosed in U.S. Pat. No. 5,729,146, entitled Quick Stacking Translator Fixture. In this patent, the spacer and rod type of assembly method is replaced with a series of one-piece stacking towers that have a series of translator plate support surfaces arranged in a stair-step fashion and separated by alignment posts. Each such support surface and corresponding adjacent alignment post is sized to support a translator plate that has been predrilled with a hole sized appropriately for its location on the stacking tower. This manner of assembling the test fixture automatically positions the translator plates at the proper, predetermined level in the text fixture. Outwardly projecting shoulders formed on the alignment posts cooperate with key slots formed on the translator plates to engage the translator plates. When the test fixture is assembled, the stacking posts are rotated to an angular position relative to the plates, causing the plates to pass under the shoulders on the stacking posts so that the shoulders all cooperate to hold the translator plates in position to provide a vertical restraint at each translator plate level.
Despite the improvements made to test fixtures over the years there remains a need for test fixtures that are assembled with precise parallel alignment between the plates. Further, there remains a distinct need for test fixtures that while meeting the requirements of precision in the assembly process are easily assembled in a minimal amount of time.
The present invention approaches the problems associated with assembling test fixtures and fixing the translator plates in position differently from previously known approaches. The test fixture of the present invention utilizes a removable loading plate having a series of loading towers spaced around the periphery of the plate. Predrilled test fixture plates are sequentially loaded onto the loading plate such that the loading towers extend through the holes in the plates. The loading towers have support steps upon which the plates rest at predetermined levels. Each test fixture plate has a series of preformed notches formed around the periphery of the plate. When all of the plates are loaded onto the loading towers, the notches in the plates align and separator posts having cooperative slots are inserted into the notches on the plates. The separator posts engage the notches to fix the plates into position. The top and bottom plates of the test fixture are secured in place with screws extending through the plates and into threaded openings in the separator posts, fixing the test fixture in precise alignment. To eliminate the problems with test fixture skewing, stabilizing plates are attached to one or more sides of the test fixture. The stabilizing plates are fixed with screws to the separator posts, which may be truncated to facilitate attachment. Because the separator posts engage the test plates at the preformed notches around the periphery of the plates rather than through predrilled holes, the stabilizing plates may be attached directly to the separator posts. At this point the loading plate and the loading towers are readily removed from the assembled test fixture.
In an alternate embodiment the test fixture plates are stacked directly onto the loading towers such that the loading towers, which have stepped shoulders, engage cooperatively formed stepped notches formed around the periphery of the plates. The stepped notches open directly to the lateral sides of the plates, allowing stacking holes to be eliminated. The test fixture plates with cooperatively formed notches are sequentially loaded onto selected loading towers with plates supported on the loading tower shoulders at the proper level. As with prior embodiments, the loading towers have support steps upon which the plates rest at predetermined levels. When all of the plates are loaded onto and supported by the loading towers, the plates are aligned and separator posts having either cooperative sequentially stepped slots or identical slots are inserted into the notches on the plates. The separator posts engage the notches to fix the plates into position. The top and bottom plates of the test fixture are then secured in place with screws extending through the plates and into threaded openings in the separator posts, fixing the test fixture in precise alignment. The loading towers are then removed and additional separator posts are inserted into the remaining groups of notches.
The test fixture of the present invention is easily assembled. For instance, the separator posts, which lock the plates in a fixed position relative to one another, simply slide into the notches in the outer periphery of the plates. Each separator post has the same configuration, so there is a need to inventory only one part for each specific test fixture arrangement. In addition, the inherent size variances associated with manufacturing tolerances from multiple spacers are eliminated by using identical, unitary support posts. This leads to a high degree of precision in assembling the test fixture. Test fixture skewing under compressive load is eliminated with the stabilizing plates.